The complex cell-cell interactions in tissue are the basis for all of the aspects of normal and pathological immune responses in vivo. These cell interactions are altered outside the context of actual tissue. We continue to develop and benchmark system of histocultures of human lymphoid tissue in vitro and use this system to study HIV pathogenesis. We have found that the histocultures of human tonsils we develop preserve both their cytoarchitecture, including a key element, a three-dimensional network of follicular dendritic cells, as well as an immune function. In vitro immunization of tonsil histocultures (but not of cells isolted from them) with tetanus toxoid (TT) results in the production of anti-TT IgG. Histocultures of lymphoid tissue are also able to support productive viral infection with two laboratory strains and six primary isolates, of syncytia-inducing (SI) and non syncytia-inducing (NSI) phenotypes, as well as macrophage- and lymphocyte-tropic viruses. Infection with HIV affects both the cellular and the functional aspects of the histocultures in an isolate-dependent manner. Infection with SI HIV-1 isolates resulted in a dramatic decline in CD4+ lymphocytes in histoculture. NSI viruses do not cause such complete depletion of tissue CD4+ lymphocytes. HIV infection greatly affected the immune response of the histocultures to tetanus toxoid. The immune response to TT was dramatically inhibited in histocultures infected with T-tropic HIV isolate. In contrast, infection with M-tropic isolate appears to lead to the enhanced production of antibodies against tetanus toxoid. Mechanisms of HIV-induced dysregulation at various stages of HIV-disease can now be studied in the context of a complex system of cell-cell interactions under controlled conditions in vitro.